Changes in temperature and in the operating bandwidth of a radio frequency receiver affect the operation of the receiver's squelch circuit. For example, when a two-way radio is operating at temperature extremes, the receiver's squelch circuit is prone to a phenomena known in the art as "squelch popping." Squelch popping is where the receiver's squelch opens and closes, in a very short period of time, even when no signal meeting the squelch circuit's threshold criteria is present. This phenomena causes a sound resembling a popping sound to be generated at the radio's speaker. The speaker popping sounds cause annoyance to radio users. The usual solution to this annoying unsquelching problem is for the radio user to increase the squelch circuits threshold level either manually, if the radio has a squelch adjustment control, or by reprogramming the radio's squelch threshold level(s) if the radio does not have a manual squelch control.
The reason that the squelch circuit unsquelches is that the squelch setting although set at a point which would provide good receiver operation during normal operating conditions, the setting is affected by extreme operating temperature changes due to the fact that temperature affects the squelch circuit components. The change in receiver operating temperature to a higher or lower operating temperature extreme, has the affect of prematurely unsquelching the circuit, or causing the squelch circuit to require a higher level input signal in order to unsquelch the circuit. The affect on the squelch circuit will depend on the temperature characteristics of the circuit.
The problem with increasing the radio's squelch threshold level (the signal input level required to unsquelch the radio) in order to avoid premature unsquelching is that it causes the radio to have reduced communication range. The decrease in communication range is due to the fact that a larger received signal level must be present at the receiver's front-end in order for the radio to unsquelch.
Since receivers used in two-way radios are usually operated over varying ambient temperature conditions, a radio user will usually be forced into having to set a "higher" (a point which would require a higher received signal to unsquelch the receiver squelch circuit) squelch setting in situations (e.g., normal operating temperatures) where the radio user could have had a lower squelch setting which would have allowed for greater operating range. This would be the case where the user has to spend a portion of his operating time in extreme temperature conditions, where squelch popping occurs and the radio user is forced to increase the radio's squelch threshold level. Typically, a radio user will tend to forget to readjust the radio's squelch control level once the user moves back into more normal operating conditions. This is especially true for radios which do not provide for manual squelch adjustment by the radio user (e.g., sophisticated radios which program squelch threshold level settings into the radio). The end result being that the radio user has a squelch threshold setting which may be too high for normal use causing otherwise good messages to go unheard.
The same unsquelching problems occurs when a radio begins receiving information signals having different channel spacings that the signals previously received. For example, in modern radios where it is common to have one or more channels in a receiver receiving information having a certain channel spacing and another set of channels receiving information having a second channel spacing specification. A specific example of this would be a 800 Megahertz band radio which could have a set of radio channels in the 821-824 MHz range using 12.5 KHz channel spacing, while having another set of radio channels operating in the 850-870 MHz range using 25 KHz channel spacing. The change in channel spacing causes a change in the characteristic noise footprint of the receiver, which is caused by the changes in deviation due to the channel spacing differences. This again causes the radio's squelch circuit to unsquelch at different threshold levels.
A need thus exists for a receiver squelch circuit which can automatically compensate for the affects caused by changes in temperature or changes in operating bandwidth. The squelch circuit should also provide for maximum receiver operating range, while at the same time being able to avoid squelch popping or premature receiver unsquelching.